Synthetic resin insulated electric circuit element



Nov. 29, 1955 J. w. SCHELL 2,725,312

SYNTHETIC RESIN INSULATED ELECTRIC CIRCUIT ELEMENT Filed Dec. 28, 1951 IN VEN TOR.

w W bw- United States Patent SYNTHETIC RESIN INSULATED ELECTRIC CIRCUIT ELEMENT .Iohn Schell, Erie, Pa., assignor to Erie Resistor Corporation, Erie, Pa., a corporation of Pennsylvania Application December 28, 1951, Serial No. 263,899

3 Claims. (Cl. 117-218) In electrical components such as resistors, capacitors, and inductances, there is need for a moisture-proof insulating coating which will stand much higher than normal operating temperatures e. g. 400 F. and at the same time will provide satisfactory protection at the normal temperatures. This invention is intended to provide such a coating. It comprises a base coat of monochlorotrifiuoethylene which provides the electrical insulation, but not the moisture barrier, and a cover coat of silicone varnish which provides the moisture barrier. The adherence of the cover coat is obtained by a non-ionic wetting agent selected from one of the higher alcohols which reduces the surface contact angle between the base and cover coats so that the cover coat will adhere uniformly when baked.

In the drawing, the single figure is a section through an electric circuit component to which the coating is applicable.

Referring to the drawing, the coating is shown applied to a tubular capacitor selected as typical of the circuit components. The capacitor has a tubular dielectric 1 with inner and outer electrode coatings 2 and 3 connected to cup-shaped terminals 4 and 5 having fixed thereto leads 6 and 7. The terminals are ordinarily soldered to the electrodes. The parts so far described are or may be of common construction.

While such components can be designed for operation at above normal temperatures, e. g. 400 F., difiiculty has been experienced in obtaining a moisture-proof coating which would protect the component at normal temperatures and which would not break down at the ele vated temperatures. At the elevated temperatures, of course there is no need for a moisture-proof coating, as

any moisture will be rapidly driven off. However, if

the coating should disintegrate or lose its effectiveness at the elevated temperatures, the component might be damaged by moisture absorption while the component was stored under humid conditions.

To provide the high temperature electrical insulation a base coat 8 of monochlorotrifluoroethylene is applied. Conveniently the base coat is applied by dipping the component in a solution of the monochlorotrifiuoroethylene in which has been dispersed sufficient inert filler to give the requisite body to the coating. The solvent for the monochlorotrifiuoroethylene is not critical, nor is the amount of filler. A non-ionic solvent is desirable in case traces of the solvent remain in the finished coating. A solvent which ionizes would cause break down of the coating, if moisture should break through to the base coating. The filler likewise it not critical. A non-nutrient filler is desirable if fungus resistance is necessary. By way of example, the base coat solution may consist of 25% water ground mica filler and 75% of a 56% suspension of monochlorotrifluoroethylene in xylol, both percentages by weight. This coating may be thinned to the desired dipping consistenecy with xylol. After dipping, the component is air dried and then is polymerized by baking at from 400 F. to 450 F., polymerizing the ice monochlorotrifluoroethylene. If the base coat is not of the desired thickness, an additional dip and baking operation may be had. The base coat provides the desired electrical resistance, but does not provide a satisfactory moisture barrier. The base coat is stable at operating temperatures up to 400 P. which is substantially above the normal operating range.

As a moisture-barrier, there is applied an exterior cover coat 9 of a silicone varnish. Such varnishes have m'oisture resistance, but do not have satisfactory electrical resistance. Furthermore, such varnishes are diificult to apply because they do not wet. To achieve wetting of the silicone varnish outer coating, a non-ionic wetting agent selected from one of the higher alcohols is mixed into the silicone varnish. This wetting agent reduces the surface contact angle between the monochlorotrifioro ethylene polymer and the silicone varnish. Without the wetting agent, the silicone varnish will not adhere to the under coat. On a fiat surface the silicone varnish will collect in drops. On inclined surfaces, silicone varnish will run oif. However, with the wetting agent, the silicone varnish will adhere. The silicone varnish may also contain a filler, (non-nutrient, if fungus resistance is required) and may be thinned with a non-ionic solvent. By way of example, the silicone varnish may consist of of Dow Corning 996 polymethylsiloxane varnish (65% solids in xylol), 24% of water ground mica filler, and 1% of alkyl aryl polyether alcohol as a wetting agent, all percentages by weight. The silicone varnish may be thinned to the desired consistency with xylol. The particular silicone varnish, higher alcohol, or filler is not critical. All silicone varnishes are non-Wetting. All higher alcohols are wetting agents. The fillers are inert. The exterior coating is applied by dipping the components into the varnish, air drying to remove the solvent, and then curing in a circulating air oven at 300 F. to polymerize the silicone. If desired, the'outer coating may be built up in two or more coats, each of which is cured in place. The polymerized silicone varnish outer coating is essentially a moisture barrier which has sufficient high temperature stability to remain effec tive over long periods of time. The non-ionic wetting agent makes the silicone varnish compatible with the base coat. Traces of the wetting agent may remain in the polymerized silicone varnish. Some of the wetting agent is driven off during the polymerization and all, or nearly all of it, may be driven off without affecting the adherence of the coatings. By bringing the silicone varnish into wetting contact with the base coat, a condition for physical adherence is preserved as the silicone varnish is polymerized and once the polymerization has taken place, the wetting agent is no longer necessary.

What is claimed as new is:

1. An electric circuit element having thereon a moisture-proof insulating coating comprising a base coat layer of polymerized monochlorotrifluoroethylene and a cover coat layer of water free silicone varnish overlying the polymerized base coat and polymerized in situ, the silicone varnish having a minor proportion of a non-ionic higher alcohol wetting agent reducing the contact angle between the polymerized base coat and the unpolymerized cover coat.

2. An electric circuit element having thereon a moisture-proof insulating coating comprising a base coat layer of polymerized monochlorotrifluoroethylene and a cover coat layer of water free silicone varnish overlying the polymerized base coat and polymerized in situ, the silicone varnish having a minor proportion of alkyl aryl polyether alcohol reducing the contact angle between the polymerized base coat and the unpolymerized cover coat.

3. An electric circuit element having thereon a moisture-proof insulating coating comprising a base coat layer of polymerized monochlorotrifiuoroethylene and a cover coat layer of polymethylsiloxane silicone varnish overlying the polymerized water free base coat and polymerized in situ, the silicone varnish having a minor proportion of a non-ionic higher alcohol wetting agent reducing the contact angle between the polymerized base coat and the unpolymerized cover coat.

References Cited in the file of this patent UNITED STATES PATENTS 2,390,370 Hyde Dec. 4, 1945 2,392,805 Biefeld Jan. 15, 1946 2,443,067 Burns June 8, 1948 4 2,459,653 Keyes Jan. 18, 1949 2,516,030 Swiss July 18, 1950 2,542,069 Young Feb. 20, 1951 2,542,071 Sprung Feb. 20, 1951 2,567,804 Davies Sept. 11, 1951 OTHER REFERENCES New High Temperature Thermoplastic, Modern Plastics, Oct. 1948.

Versatile Fluorine Plastics, Chemical and Engineering News, vol. 30, No. 26, June 30, 1952, pp. 2688-2691.

Fluorinated Plastics, British Plastics, vol. 21, No. 243, Aug. 1949, pp. 457-465. 

1. AN ELECTRICAL CIRCUIT ELEMENT HAVING THEREON A MOISTURE-PROOF INSULATING COATING COMPRISING A BASE COAT LAYER OF POLYMERIZATION MONOCHLOROTRIFLUOROETHYLENE AND A COVER COAT LAYER OF WATER FREE SILICON VARNISH OVERLYING THE POLYMERIZED BASE COAT AND A POLYMERIZED IN SITU, THE SILI- 